1. Field of the Invention
This invention relates to the field of chromatography generally, and more specifically to the detection of anions in the presence of an excess of another anion, such as in liquids used in semiconductor manufacturing.
2. Related Art
Typical known methods of determining the presence of an anion in liquid samples use wet-chemistry procedures, which are time consuming and which typically detect and/or quantify only one analyte at a time. Some ion-chromatographic methods have been developed to address this determination, but detection has typically been by conductivity, absorbance, or amperometry.
Chromatography is the separation of mixtures of similar species (e.g., sugars, proteins, anions). In the technique, there are two “work horses”: 1) a stationary phase; and 2) a mobile phase. As the names indicate, the first phase does not move but the second one does; both phases are selected because they will interact with a mixture's species (also known as analytes). Some of the analytes prefer to be in the mobile phase and thus move together out of the system rapidly; other species would rather interact with the stationary phase, moving along much more slowly. It is this difference in preferences that allows the mixture to be separated. Chromatographic systems can be designed very reproducibly, meaning that each mixture component will take a specific amount of time to move through the system. This retention time, as it is called, is the main way that substances are identified; if a sample component's retention time matches that of a known standard, then the two substances are assumed to be the same. Quantitation is achieved by measuring the responses of known concentrations of components, and comparing these numbers with sample data.
One large class of chromatographic techniques is known as liquid chromatography (LC). Here, the stationary phase is a sand-like substance called a resin, which has been treated with chemicals to establish specific separating properties. The resin is packed in a thin, cylindrical column of, typically, plastic or metal. (Although the term “column” is used herein, the skilled artisan will recognize “column” to be a generic term for any separating means.) The mobile phase is a liquid that flows through this column at all times during operation. Ion chromatography (IC) is a special type of LC. Here, the anions or cations in a mixture are separated, using specific resins and mobile phases (called eluents). The eluents usually are either bases (for anion separations) or acids (for cation samples).
The key deficiency with typical ion-chromatographic methods is that there is at least some doubt about the identity of each sample component. There may be an unknown substance that has the same retention properties as one of the analytes. In such a case, retention time alone is not sufficient to establish identification. Another detection technique, known as mass spectrometry, can help in these situations. Here, a substance is ionized so that it will break apart into fragments. The fragmentation pattern for each chemical is distinct and reproducible from one analysis to the next. These facts make mass spectrometry one of the best means of identifying a substance. (Because a mass spectrometer can help detect, identify, and quantify specific substances, it is often called a specific detector.)
Mass spectrometry can be linked with a chromatographic system like IC. The chromatography portion (i.e., the separation) is accomplished first. As the eluent exits the ion chromatograph, the liquid is directed into the spectrometer for fragmentation and identification of each component. The resulting pattern can often confirm if each substance is the same as what the retention time indicates.
One common problem in the semiconductor art and other arts is that a user of a liquid chemical would like to detect and/or quantify one or more anions, which exists in the solution in a very minor portion in a huge sea of anions of another type. For example, aqueous acidic solutions used in semiconductor manufacturing may contain very minor portions of chloride ion (Cl−) in a very major portion of nitrate ions (NO3−). The same situation can occur in basic pH solutions. The problem is particularly acute in strong acid and strong basic solutions. In these situations, the strong acids and strong bases are completely dissociated, thereby providing the extremely high concentration of the major-portion anion. An example of this is aqueous hydrochloric acid (HCl), which is dissociated into positively charged hydronium ions and negatively charged chloride ions. It may be necessary or convenient to the user to know how much sulfate ion (SO42−) is contained within an aqueous hydrochloric-acid solution.
A combination of liquid chromatography and mass spectrometry is presently used for the detection and/or quantification of certain species in so-called “neutral organics” such as drug compositions, protein compositions, and the like. However, it does not appear anywhere in the art known to the inventor to use a combination of ion chromatography with mass spectrometry to detect and/or quantify a very minor portion of an anion in a very major portion of another anion, particularly in weak acids, strong acids, and salts thereof.